1. Field of the Invention
This invention relates to the measurement of magnetic properties and, in particular, to the measurement of magnetic properties of materials used in devices relying on movement of single wall magnetic domains.
2. Art Background
The performance of a device based on the movement of single wall magnetic domains through a material capable of supporting such domains, e.g., one with a garnet crystal structure, depends, to a significant extent, on the properties of the material. Among these properties is the coercivity of a single wall magnetic domain in the magnetic material, i.e., the difficulty of initiating the movement of the domain in the material. To insure the manufacture of devices having suitable properties and to insure the consistent functioning of circuits incorporating these devices, it is desirable to measure the coercivity of the domain supporting material during the manufacture process to determine whether it is sufficiently low to prevent unacceptable degradation of the properties of the finished device.
A variety of methods has been developed for measuring the coercivity of a domain supporting material, e.g., a garnet layer, utilized in a device. The most common method for measuring coercivity involves the measurement of the response to a stripe domain pattern to an oscillating magnetic field. In this approach, a random demagnetized stripe domain pattern, such as shown in FIG. 1, is employed. This domain pattern is subjected to an oscillating magnetic field and the extent of the modulation of the stripe width for a given field is measured. The data obtained are then plotted and the value of the field obtained by extrapolation to zero modulation of the domain width is defined as the coercivity of the magnetic garnet layer. (See R. D. Pierce, Journal of Crystal Growth, 27,299 (1974).) An alternative technique for measuring coercivity involves the monitoring of bubble translation in a pulsed field gradient. In this technique, single wall magnetic domains, i.e., bubbles, are produced in the domain supporting material. A pulsed field gradient is then repetitively applied to the material and its magnitude is increased until a bubble begins to move. The field at which translation initiates is considered the coercivity. (See Vella-Coleiro and Tabor, Applied Physics Letters, 27, 7 (1972).)
Each of the previously mentioned techniques has certain advantages and certain shortcomings. The oscillating stripe domain technique is an expedient method of measuring coercivity. However, this technique suffers from a large experimental error--generally greater than 50 percent. On the other hand, the bubble translation technique is an accurate method of measuring coercivity. However, this method is extremely tedious.
Neither of the previously mentioned techniques used for measuring coercivity is adaptable for quality control measurements. The oscillating stripe domain procedure is relatively fast and, therefore, is usable to measure the coercivity of a reasonable sampling of materials as they are used to make devices in a manufacturing facility. However, the inaccuracies inherent in this technique make it impossible to insure that the desired coercivities are being obtained. On the other hand, the bubble translation technique is too slow to be satisfactory for quality control measurements. Thus, as yet a coercivity measuring technique suitable for testing the reproducibility of coercivity in a production environment, is not available.